Boiling point reactor with heat removal

ABSTRACT

In a boiling point reactor wherein reactants react exothermically to vaporize a portion of the reaction mixture in a fixed catalyst bed, a cooling coil is provided within the bed which condenses a portion of the vaporized reaction mixture. The condensing heat transfer coefficient within the bed is thus increased dramatically and efficient heat removal is achieved.

BACKGROUND OF THE INVENTION

Temperature control for exothermic reactions has been achieved in thepast in a number of ways. One such method has been to place coolingcoils within the catalyst bed to remove the heat of reaction. This typeof reactor has commonly been referred to as an adiabatic reactor. Whilethis arrangement is useful on a small scale, larger reactors andcatalyst beds present problems. The low liquid velocities required toachieve the desired residence times in the catalyst bed provide verypoor heat transfer coefficients and inefficient cooling.

Another method used, especially where one of the reactants is gaseous,is to introduce cold reactant or "quenches" along the length of thecatalyst bed. This type of cooling has been effectively used in suchreactors as hydrocrackers where large quantities of hydrogen are needed.The amount of quench used is limited by compressors and other relatedgas recycle equipment. Additionally, distribution of the cold gas inlarger catalyst beds can create such problems as hot spots.

The adiabatic and quench reactors noted above remove only sensible heatin the reactors and are thus limited by the specific heats of thereactants or quench gases. More efficient temperature control can beachieved in a boiling point or isothermal reactor where the reaction iscarried out at a pressure so as to cause vaporization of a portion ofthe reaction mixture. The latent heat of vaporization absorbs theexothermic heat of reaction and limits the temperature rise in the bed.This, too, has limitations in that in highly exothermic reactions all ofthe reaction mixture may be vaporized before the desired conversion ofreactants is achieved. To solve this problem several reactors may beused in series with the effluent from one reactor being condensed beforeintroduction to the next reactor. In this type of reactor the entirereaction mixture passes through and out of the reactor and containsreactants, products, liquids and gases.

A given composition, the reaction mixture, will have a different boilingpoint at different pressures, hence the temperature in the reactor iscontrolled by adjusting the pressure to the desired temperature withinthe recited range. The boiling point of reaction mixture thus is thetemperature of the reaction and the exothermic heat of reaction isdissipated by vaporization of the reaction mixture. The maximumtemperature of any heated liquid composition will be the boiling pointof the composition at a given pressure, with additional heat merelycausing more boil up. The same principal operates in the presentinvention to control the temperature. There must be liquid present,however, to provide the boil up, otherwise the temperature in thereactor will continue to rise until the catalyst is damaged. This is asubstantial departure from the prior art for this type of reactor, wheresufficient pressure was employed to maintain the reaction mixture inliquid phase.

The present invention which relates to the liquid phase type of reactionalso provides means for removing heat from the fixed continuous catalystbed. These and other advantages will become apparent from the followingdescriptions.

SUMMARY OF THE INVENTION

The present invention provides a means of efficient heat removal in thecatalyst bed of a boiling point reactor. Cooling coils are placed withinthe bed at intervals to condense a portion of the vaporized reactionmixture. Because the cooling coils are condensing the mixture in thebed, heat transfer is increased dramatically over conventional adiabaticreactors. Thus efficient heat removal in packed catalyst beds isachieved. In a conventional non isothermal reactor, i.e., liquid phasereactors, the cooling coils are removing only sensible heat of reaction,whereas in the present invention since there are boiling components,condensation heat transfer is obtained. In one embodiment of theinvention several cooling coils are arranged in a large catalyst bed ina downflow boiling point reactor such that each cooling coil iscondensing the vapor from a point above the coil within the bed. Toachieve high heat transfer coefficients, the coils should be in a packedportion of the bed to insure the necessary vapor velocities whichimprove the condensing heat transfer coefficients.

In a second embodiment several beds of catalyst may be placed in thereactor separated by inert packing. The cooling coils are placed in theinert packing to condense the vapor from the catalyst bed directlyabove.

The catalyst bed may be described as a fixed continuous bed, that is,the catalyst is loaded into the reactor in its particulate form to fillthe reactor or reaction zone, although there may be one or more suchcontinuous beds in a reactor, separated by spaces devoid of catalystand/or inert particulate material.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of one embodiment of the presentinvention.

FIG. 2 is a schematic representation of a second embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 there is shown a boiling point reactor, generally indicated at10, which is filled with catalyst 16 appropriate for carrying out anexothermic reaction. The reactants enter the reactor through inlet 12.

The temperature in the reactor is thus controlled by the pressure used.The temperature in the reactor and catalyst bed is limited to theboiling point of the mixture present at the pressure applied,notwithstanding the magnitude of the exotherm. A small exotherm maycause only a few percent of the liquid in the reactor to vaporizewhereas a large exotherm may cause 30-90% of the liquids to vaporize.The temperature, however, is not dependent on the amount of materialvaporized but the composition of the material being vaporized at a givenpressure. That "excess" heat of reaction merely causes a greater boil up(vaporization) of the material present. As the reactants flow downwardlythrough the catalyst bed 16, the heat of reaction is absorbed by thereaction mixture which causes a portion of the mixture to vaporize. Themixed gas and liquid phases pass downward through the bed and contactsfirst cooling coil 18a where the gaseous portion of the mixture iscondensed. The condensation cooling provides efficient heat transfercoefficients and effective removal of a portion of the latent heat ofcondensation of the gases.

The reaction mixture containing cooled material from the first coolingcoil 18a passes down through the bed 16 where further reaction andvaporization occur. The new reaction mixture then contacts secondcooling coil 18b where the vapors formed are again condensed removing aportion of the latent heat of condensation.

The reaction mixture than again proceeds through the catalyst bed 16reacting and vaporizing a portion of the mixture which is contacted bythird cooling coil 18c and a third time a portion of the latent heat ofcondensation is removed. The length of the bed and number of coils maybe adjusted to achieve the desired conversion of reactants with thefinal mixture exiting the reactor through outlet 14.

Liquid redistribution trays (not shown) may be desirable at differentlocations within the bed to prevent channeling. An appropriate placementof some of the redistribution trays would be directly below thecondensing coils where the volume of liquid is suddenly increased.

Because the concentration of reactants is decreasing due to consumptionby the reaction as the mixture flows downward through the bed, therewill be less heat generated and consequently less of the mixture will bevaporized. Thus, the size and location of the coils within the bed canbe adjusted to provide maximum heat removal and the most efficienttemperature control.

Referring now to FIG. 2 an alternative embodiment of the presentinvention is shown in schematic form. The reactor, generally indicatedat 20, is shown to contain two catalyst beds 23a and 23b separated bybeds of inert packing 24a and 24b. The catalyst beds 23a and 23b areshown to be supported by trays 26a and 26b which may also act asredistribution trays. Inert packing in bed 24a is shown supported by aliquid distribution tray 27a.

The reactants enter the reactor 20 through inlet 21 and are passed oversuitable catalyst 23a where a portion of reactants react exothermicallypartially vaporizing the resultant reaction mixture. The two phasereaction mixture then passes downward through a cooling bed containinginert packing 24a where it is contacted with cooling coil 25a in theinert packing bed. The inert packing insures that the vapor velocity issufficiently high to improve the condensate heat transfer coefficient tosubstantially condense the vapor in the mixture. Liquid distribute tray27a supports the inert packing 24a and prevents channeling of the nowliquid reaction mixture. A second catalyst bed 23b provides for furtherreaction and partial vaporization of the reaction mixture. Thealternating catalyst bed/cooling bed arrangement may be multiplied untilthe desired conversion of reactants is achieved with the final productwithdrawn through outlet 22.

The invention claimed is:
 1. In a boiling point reactor whereinreactants react exothermically to vaporize a portion of the reactionmixture in a vertically disposed fixed bed, wherein the improvementcomprises providing as said vertically disposed fixed bed a plurality ofalternating particulate catalyst beds and inert packing beds having acooling coil within each of said inert packing beds for removing heat bycondensing a portion of the vaporized reaction mixture.